Plasma generation and processing with multiple radiation sources

ABSTRACT

Plasma-assisted methods and apparatus that use multiple radiation sources are provided. In one embodiment, a plasma is ignited by subjecting a gas in a processing cavity to electromagnetic radiation having a frequency less than about 333 GHz in the presence of a plasma catalyst, which may be passive or active. A controller can be used to delay activation of one radiation source with respect to another.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Priority is claimed to U.S. Provisional Patent Application No.60/378,693, filed May 8, 2002, No. 60/430,677, filed Dec. 4, 2002, andNo. 60/435,278, filed Dec. 23, 2002, all of which are fully incorporatedherein reference.

FIELD OF THE INVENTION

[0002] The invention relates to methods and apparatus forplasma-assisted processing, and in particular to using multipleelectromagnetic radiation sources in combination.

BACKGROUND OF THE INVENTION

[0003] It is known that a single microwave radiation source can be usedto generate a plasma by subjecting a gas to a sufficient amount ofmicrowave radiation. A single microwave energy source can be damaged,however, when one source directs microwave energy into a plasma chamberthat reflects the energy back toward the same source. This can beespecially problematic when there is no strong microwave absorber in thecavity, such as when a plasma is not yet formed. Also, multiplemicrowave energy sources are particularly susceptible to damage whencombined to ignite or sustain a plasma. For example, a first source canbe damaged when it directs microwave energy into a chamber because thatenergy could be directed into another simultaneously connected radiationsource.

[0004] It is also known that plasma ignition is usually easier at gaspressures substantially less than atmospheric pressure. However, vacuumequipment, which is required to lower the gas pressure, can beexpensive, as well as slow and energy-consuming. Moreover, the use ofsuch equipment can limit manufacturing flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION

[0005] Consistent with the present invention, apparatus and methodsusing multiple radiation sources (e.g., microwave radiation sources) areprovided. Plasmas formed from gases at pressures at about one atmosphereor higher can strongly absorb microwave radiation. Strong absorption canbe used to reduce the possibility of damage to a particular source byits own radiation that may be reflected back or radiation from othersources. Consequently, high power plasma-assisted processes can becarried out using multiple (e.g., low power) sources coupled to the sameplasma.

[0006] In one embodiment, a radiation apparatus may include a cavity.The radiation apparatus can also include a first high-frequencyradiation source and a second high-frequency radiation source fordirecting radiation into the cavity. The radiation apparatus may includea controller for sequentially activating the second radiation sourceafter the first radiation source is activated.

[0007] In another embodiment consistent with this invention, a plasmafurnace may include a chamber, a conduit for supplying gas to thechamber, a plurality of radiation sources arranged to radiate radiationinto the chamber, and a controller for delaying activation of all but afirst of the plurality of radiation sources until after the firstradiation source is activated. A radiation apparatus and a plasmafurnace consistent with this invention may include a plasma catalystlocated proximate to the cavity. The plasma catalyst can cooperate withthe microwave radiation in the presence of a gas to form a plasma. Thecatalyst can be passive or active. A passive plasma catalyst can includeany object capable inducing a plasma by deforming a local electric field(e.g., an electromagnetic field) consistent with this invention, withoutnecessarily adding additional energy. An active plasma catalyst, on theother hand, is any particle or high energy wave packet capable oftransferring a sufficient amount of energy to a gaseous atom or moleculeto remove at least one electron from the gaseous atom or molecule in thepresence of electromagnetic radiation. In both cases, a plasma catalystcan improve, or relax, the environmental conditions required to ignite aplasma.

[0008] In another embodiment consistent with this invention, a methodfor forming a plasma is provided. The method can include employing atleast first and second radiation sources arranged to direct radiationinto a processing or heating region. The method can include introducinggas into the region, activating the first radiation source to facilitateformation of plasma in the heating region, and activating the secondradiation source after the plasma is formed.

[0009] In yet another embodiment consistent with this invention,additional methods and apparatus are provided for forming a plasma usinga dual-cavity system. The system can include a first ignition cavity anda second cavity in fluid communication with each other. The method caninclude (i) subjecting a gas in the first ignition cavity toelectromagnetic radiation having a frequency less than about 333 GHz,such that the plasma in the first cavity causes a second plasma to formin the second cavity, and (ii) sustaining the second plasma in thesecond cavity by subjecting it to additional electromagnetic radiation.

[0010] Additional plasma catalysts, and methods and apparatus forigniting, modulating, and sustaining a plasma consistent with thisinvention are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Further aspects of the invention will be apparent uponconsideration of the following detailed description, taken inconjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

[0012]FIG. 1A shows a schematic diagram of an illustrative apparatusthat includes multiple radiation sources consistent with this invention;

[0013]FIG. 1B shows a flow chart for an illustrative method consistentwith this invention;

[0014]FIG. 2 shows a simplified illustrative embodiment of a portion ofa plasma system for adding a powder plasma catalyst to a plasma cavityfor igniting, modulating, or sustaining a plasma in a cavity consistentwith this invention;

[0015]FIG. 3 shows an illustrative plasma catalyst fiber with at leastone component having a concentration gradient along its lengthconsistent with this invention;

[0016]FIG. 4 shows an illustrative plasma catalyst fiber with multiplecomponents at a ratio that varies along its length consistent with thisinvention;

[0017]FIG. 5A shows another illustrative plasma catalyst fiber thatincludes a core underlayer and a coating consistent with this invention;

[0018]FIG. 5B shows a cross-sectional view of the plasma catalyst fiberof FIG. 5A, taken from line 5B-5B of FIG. 5A, consistent with thisinvention;

[0019]FIG. 6 shows an illustrative embodiment of another portion of aplasma system including an elongated plasma catalyst that extendsthrough an ignition port consistent with this invention;

[0020]FIG. 7 shows an illustrative embodiment of an elongated plasmacatalyst that can be used in the system of FIG. 6 consistent with thisinvention;

[0021]FIG. 8 shows another illustrative embodiment of an elongatedplasma catalyst that can be used in the system of FIG. 6 consistent withthis invention; and

[0022]FIG. 9 shows an illustrative embodiment of a portion of a plasmasystem for directing ionizing radiation into a radiation chamberconsistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] Consistent with the present invention, plasma apparatus andmethods that use multiple radiation sources are provided. In oneembodiment, as shown in FIG. 1A, a radiation apparatus may includecavity 12. Further, in one embodiment, the radiation apparatus mayfurther include a plasma catalyst located proximate to the cavity, whichmay cooperate with the radiation to cause the gas to form a plasma.

[0024] This invention may further relate to methods and apparatus forinitiating, modulating, and sustaining a plasma for a variety ofapplications, including heat-treating, synthesizing and depositingcarbides, nitrides, borides, oxides, and other materials, doping,carburizing, nitriding, and carbonitriding, sintering, multi-partprocessing, joining, sintering, decrystallizing, making and operatingfurnaces, gas exhaust-treating, waste-treating, incinerating, scrubbing,ashing, growing carbon structures, generating hydrogen and other gases,forming electrodeless plasma jets, plasma processing in assembly lines,sterilizing, etc.

[0025] In another embodiment, a plasma furnace is provided that mayinclude a chamber, a conduit for supplying gas to the chamber, aplurality of radiation sources arranged to radiate radiation into thechamber and a controller for delaying activation of all but a first ofthe plurality of radiation sources until after the first radiationsource is activated. Each of these components is explained more fullybelow.

[0026] This invention can be used for controllably generating heat andfor plasma-assisted processing to lower energy costs and increaseheat-treatment efficiency and plasma-assisted manufacturing flexibility.

[0027] Therefore, a plasma catalyst for initiating, modulating, andsustaining a plasma is provided. The catalyst can be passive or active.A passive plasma catalyst can include any object capable of inducing aplasma by deforming a local electric field (e.g., an electromagneticfield) consistent with this invention without necessarily addingadditional energy through the catalyst, such as by applying a voltage tocreate a spark. An active plasma catalyst, on the other hand, may be anyparticle or high energy wave packet capable of transferring a sufficientamount of energy to a gaseous atom or ion to remove at least oneelectron from the gaseous atom or molecule, in the presence ofelectromagnetic radiation.

[0028] The following commonly owned, concurrently filed U.S. patentapplications are hereby incorporated by reference in their entireties:U.S. patent application Ser. No. ______ (Atty. Docket No. 1837.0008),Ser. No. ______ (Atty. Docket No. 1837.0009), Ser. No. ______ (Atty.Docket No. 1837.0010), Ser. No. ______ (Atty. Docket No. 1837.0011),Ser. No. ______ (Atty. Docket No. 1837.0012), Ser. No. ______ (Atty.Docket No. 1837.0013), Ser. No.______ (Atty. Docket No. 1837.0015), Ser.No. ______ (Atty. Docket No. 1837.0016), Ser. No. ______ (Atty. DocketNo. 1837.0017), Ser. No. ______ (Atty. Docket No. 1837.0020), Ser. No.______ (Atty. Docket No. 1837.0021), Ser. No.______ (Atty. Docket No.1837.0023), Ser. No. ______ (Atty. Docket No. 1837.0024), Ser. No.______ (Atty. Docket No. 1837.0025), Ser. No.______ (Atty. Docket No.1837.0026), Ser. No. ______ (Atty. Docket No. 1837.0027), Ser. No.______ (Atty. Docket No. 1837.0028), Ser. No.______ (Atty. Docket No.1837.0029), Ser. No. ______(Atty. Docket No. 1837.0030), Ser. No.______(Atty. Docket No. 1837.0032), and Ser. No. ______ (Atty. DocketNo. 1837.0033).

[0029] Illustrative Plasma System

[0030]FIG. 1A shows a schematic diagram of an illustrative radiationapparatus consistent with one aspect of the invention. The exemplaryradiation apparatus may include cavity 12 formed in a vessel that may bepositioned inside a microwave chamber (also known as applicator) 14. Inanother embodiment (not shown), vessel 12 and microwave chamber 14 arethe same, thereby eliminating the need for two separate components. Thevessel in which cavity 12 is formed can include one or moreradiation-transmissive insulating layers to improve its thermalinsulation properties without significantly shielding cavity 12 from theradiation.

[0031] In one embodiment, the radiation apparatus may be configured as aplasma furnace. One skilled in the art will appreciate that theradiation apparatus may also be used for initiating, modulating, andsustaining a plasma for a variety of other applications, including, forexample, heat-treating, synthesizing and depositing carbides, nitrides,borides, oxides, and other materials, doping, carburizing, nitriding,and carbonitriding, sintering, multi-part processing, joining,sintering, decrystallizing, making and operating furnaces, gasexhaust-treating, waste-treating, incinerating, scrubbing, ashing,growing carbon structures, generating hydrogen and other gases, formingelectrodeless plasma jets, plasma processing in assembly lines,sterilizing, etc.

[0032] In one embodiment, cavity 12 is formed in a vessel made ofceramic. Due to the extremely high temperatures that can be achievedwith plasmas consistent with this invention, a ceramic capable ofoperating at about 3,000 degrees Fahrenheit can be used. The ceramicmaterial can include, by weight, 29.8% silica, 68.2% alumina, 0.4%ferric oxide, 1% titania, 0.1% lime, 0.1% magnesia, 0.4% alkalies, whichis sold under Model No. LW-30 by New Castle Refractories Company, of NewCastle, Pa. It will be appreciated by those of ordinary skill in theart, however, that other materials, such as quartz, and those differentfrom the one described above, can also be used consistent with theinvention.

[0033] A plasma may be formed in a partially open cavity inside a firstbrick and topped with a second brick. The cavity may have dimensions ofabout 2 inches by about 2 inches by about 1.5 inches. At least two holesmay also be provided in the brick in communication with the cavity: onefor viewing the plasma and at least one hole for providing the gas. Thesize of the cavity can depend on the desired plasma process beingperformed. Also, the cavity should at least be configured to prevent theplasma from rising/floating away from the primary processing region.

[0034] Cavity 12 can be connected to one or more gas sources 24 (e.g., asource of argon, nitrogen, hydrogen, xenon, krypton) by line 20 andcontrol valve 22, which may be powered by power supply 28. Line 20 maybe tubing (e.g., between about {fraction (1/16)} inch and about{fraction (1/4)} inch, such as about {fraction (1/8)}″). Also, ifdesired, a vacuum pump can be connected to the chamber to remove anyacidic fumes that may be generated during plasma processing. Also, anyexcess gas may escape the chamber via gas port 13 or similar additionalgas ports.

[0035] A radiation leak detector (not shown) may be installed nearsource 26 and waveguide 30 and connected to a safety interlock system toautomatically turn off the radiation (e.g., microwave) power supply if aleak above a predefined safety limit, such as one specified by the FCCand/or OSHA (e.g., 5 mW/cm²), was detected.

[0036] In one embodiment, the radiation apparatus may include radiationsource 26 for directing radiation into the cavity. The radiationapparatus may further include radiation source 27 for directingradiation into the cavity. Although FIG. 1A depicts two radiationsources, it will be appreciated that the radiation apparatus can operatewith two or more sources.

[0037] In one embodiment, source 26 may be configured to generateradiation that is cross-polarized relative to microwave radiationgenerated by source 27 and/or any other additional sources.

[0038] Each of radiation sources 26 and 27 may be a magnetron, aklystron, a gyrotron, a traveling-wave tube amplifier/oscillator or anyother device capable of generating radiation, such as microwaveradiation. Also, the frequency of the radiation is believed to benon-critical in many applications. Thus, for example, radiation havingany frequency less than about 333 GHz can be used consistent with thisinvention. For example, frequencies, such as power line frequencies(about 50 Hz to about 60 Hz), can be used, although the pressure of thegas from which the plasma is formed may be lowered to assist with plasmaignition. Also, any radio frequency or microwave frequency can be usedconsistent with this invention, including frequencies greater than about100 kHz. In most cases, the gas pressure for such relatively highfrequencies need not be lowered to ignite, modulate, or sustain aplasma, thereby enabling many plasma-processes to occur over a broadrange of pressures, including atmospheric pressure and above.

[0039] Radiation source 26, which may be powered by electrical powersupply 28, directs radiation energy into chamber 14 through one or morewaveguides 30. It will be appreciated by those of ordinary skill in theart that source 26 can be connected directly to cavity 12 or chamber 14,thereby eliminating waveguide 30. The radiation energy entering cavity12 is used to ignite a plasma within the cavity. This plasma can besubstantially sustained and confined to the cavity by couplingadditional radiation with the catalyst. Other radiation sources (such as27) may similarly be directly connected to cavity 12 or chamber 14 orthrough one or more waveguides. Additionally, each one of them may bepowered by power supply 28 or any other combination of power suppliesmay be used.

[0040] Radiation energy, from radiation source 26, can be suppliedthrough circulator 32 and tuner 34 (e.g., 3-stub tuner). Tuner 34 can beused to minimize the reflected power as a function of changing ignitionor processing conditions, especially after the plasma has formed becausemicrowave power, for example, will be strongly absorbed by the plasma.Similarly, radiation energy from radiation source 27 may be suppliedthrough circulator 31 and tuner 33, although the use of circulators andtuners are optional.

[0041] In one embodiment, each of the radiation sources may beprotectively separated from the chamber by an isolator (not shown). Anisolator permits radiation to pass in one direction only, therebyprotecting a source not only from reflected radiation, but also fromradiation from other sources. However, consistent with this invention,reflected radiation can be minimized, especially during the early stagesof plasma ignition.

[0042] Detector 42 can develop output signals as a function of thetemperature or any other monitorable condition associated with a workpiece (not shown) within cavity 12 and provide the signals to controller44. Dual temperature sensing and heating, as well as automated coolingand gas flow controls can also be used. Also, controller 44 may beprogrammed to sequentially activate source 27 after source 26 isactivated. In another embodiment, controller 44 may delay activation ofsource 27 for a predetermined period following the activation of source26. Sources 26 and 27 may also be delayed in a similar manner and may betriggered by any measurable event, if desired.

[0043] In one embodiment, detector 42 may provide an indication ofmicrowave radiation absorption, and controller 44 may delay activationof one or more of the plurality of microwave radiation sources untilafter controller 44 receives a signal from detector 42 that apredetermined absorption threshold level has been reached.

[0044] Detector 42 may be any device that detects one or more of heat,radiation absorption, radiation reflectance, radiation transmission, theexistence of plasma, or any other phenomena signaling whether plasmaformation has or has not occurred. Examples of such detectors includeheat sensors, pyrometers, or any other sensor capable of detecting heat,temperature, radiation absorption, radiation reflectance, radiationtransmission, the existence of plasma, or any other radiation relatedphenomena.

[0045] Detector 42 can develop output signals as a function of thetemperature or any other monitorable condition associated with a workpiece (not shown) within cavity 12 and provide the signals to controller44. Dual temperature sensing and heating, as well as automated coolingrate and gas flow controls can also be used. Controller 44 in turn canbe used to control operation of power supply 28, which can have oneoutput connected to source 26 as described above and another outputconnected to valve 22 to control gas flow into cavity 12. Although notshown, controller 44 or other similar controllers may be used to controloperation of any other power supplies that may be used to supply powerto the other radiation sources.

[0046] In another embodiment, detector 42 may provide an indication ofradiation absorption by, for example, an object that is being processed.In this case, controller 44 may delay activation of subsequent sourcesuntil after controller 44 receives a signal from detector 42 that apredetermined absorption level has been reached.

[0047] Controller 44 may also be configured to delay activation of atleast one of the plurality of radiation sources for a predeterminedperiod following activation of the first radiation source. Then, each ofthe remaining plurality of radiation sources may be successivelyactivated at predetermined intervals, if desired. Controller 44 may alsobe configured to activate one or more additional radiation sources onlyafter at least one of the first and second radiation sources isactivated. Also, controller 44 may be configured to activate each of theplurality of additional radiation sources only after each of the firstand the second radiation sources is activated.

[0048] Consistent with this invention, a plasma apparatus may include aplasma catalyst that is located proximate to a plasma cavity. Thecatalyst can cooperate with the radiation to cause a gas to form aplasma. Also, as used herein, the phrase “proximate the cavity” meanseither within the cavity or at a location sufficiently close to thecavity to effect the formation of the plasma.

[0049] As explained more fully below, the location ofradiation-transmissive cavity 12 in chamber 14 may not be critical ifchamber 14 supports multiple modes, and especially when the modes arecontinually or periodically mixed. As also explained more fully below,motor 36 can be connected to mode-mixer 38 for making the time-averagedradiation energy distribution substantially uniform throughout chamber14. Furthermore, window 40 (e.g., a quartz window) can be disposed inone wall of chamber 14 adjacent to cavity 12, permitting temperaturedetector 42 (e.g., an optical pyrometer) to be used to view a processinside cavity 12. In one embodiment, the optical pyrometer output canincrease from zero volts as the temperature rises to within the trackingrange.

[0050] The invention may be practiced, for example, by employingmicrowave sources at both 915 MHz and 2.45 GHz provided byCommunications and Power Industries (CPI), although radiation having anyfrequency less than about 333 GHz can be used. The 2.45 GHz system mayprovide, for example, continuously variable microwave power from about0.5 kilowatts to about 5.0 kilowatts. A 3-stub tuner may allow impedancematching for maximum power transfer and a dual directional coupler maybe used to measure forward and reflected powers. Also, opticalpyrometers may be used for remote sensing of the sample temperature.

[0051] As mentioned above, radiation having any frequency less thanabout 333 GHz can be used consistent with this invention. For example,frequencies, such as power line frequencies (about 50 Hz to about 60Hz), can be used, although the pressure of the gas from which the plasmais formed may be lowered to assist with plasma ignition. Also, any radiofrequency or microwave frequency can be used consistent with thisinvention, including frequencies greater than about 100 kHz. In mostcases, the gas pressure for such relatively high frequencies need not belowered to ignite, modulate, or sustain a plasma, thereby enabling manyplasma-processes to occur at atmospheric pressures and above.

[0052] The equipment may be computer controlled using LabView 6isoftware, which may provide real-time temperature monitoring andmicrowave power control. Noise may be reduced by using shift registersto generate sliding averages of suitable number of data points. Also,the number of stored data points in the array may be limited to improvespeed and computational efficiency. The pyrometer may measure thetemperature of a sensitive area of about 1 cm², which may be used tocalculate an average temperature. The pyrometer may sense radiantintensities at two wavelengths and fit those intensities using Planck'slaw to determine the temperature. It will be appreciated, however, thatother devices and methods for monitoring and controlling temperature arealso available and can be used consistent with this invention. Controlsoftware that can be used consistent with this invention is described,for example, in commonly owned, concurrently filed U.S. patentapplication Ser. No. ______ (Attorney Docket No. 1837.0033), which ishereby incorporated by reference in its entirety.

[0053] Chamber 14 may have several glass-covered viewing ports withradiation shields and one quartz window for pyrometer access. Severalports for connection to a vacuum pump and a gas source may also beprovided, although not necessarily used.

[0054] The exemplary radiation apparatus may also include a closed-loopdeionized water cooling system (not shown) with an external heatexchanger cooled by tap water. During operation, the deionized water mayfirst cool the magnetron, then the load-dump in the circulator (used toprotect the magnetron), and finally the microwave chamber through waterchannels welded on the outer surface of the chamber.

[0055] Methods and Apparatus Using Multiple Radiation Sources

[0056]FIG. 1B shows a method employing at least a first and secondradiation source, both arranged to direct radiation into a plasmaformation region. The method may include introducing a gas (step 45)into a plasma-formation region. In one embodiment, this may beaccomplished by turning on valve 22 of FIG. 1A. It will be appreciatedby those of ordinary skill in the art that the plasma-formation regioncould be a cavity, which can be completely closed or partially open. Forexample, in certain applications, such as in plasma-assisted furnaces,the cavity could be entirely closed. See, for example, commonly owned,concurrently filed U.S. patent application Ser. No. ______ (AttorneyDocket No. 1837.0020), which is fully incorporated herein by reference.In other applications, however, it may be desirable to flow a gasthrough the cavity, and therefore the cavity must be open to somedegree. In this way, the flow, type, and pressure of the flowing gas canbe varied over time. This may be desirable because certain gases withlower ionization potentials, such as argon, are easier to ignite but mayhave other undesirable properties during subsequent plasma processing.

[0057] The method may further include activating a first radiationsource to facilitate formation of plasma in step 47. In one embodiment,plasma formation may be facilitated using some kind of a plasmacatalyst, such as a pointed metal tip, a spark generator, carbon,fiberous material, powderous material or any other catalyst capable offacilitating plasma ignition. Additional examples of plasma catalystsand their uses consistent with the present invention are more fullydescribed below.

[0058] The method may further include activating a second radiationsource after the plasma is formed (step 49). In one embodiment,radiation source 27 may be activated after the first radiation source isactivated. The method may further include activating at least oneadditional radiation source after at least one of the first and secondsources is activated. Further, the activation of the at least oneadditional radiation source may be delayed until after both the firstand second sources are activated.

[0059] The radiation sources may be, for example, a magnetron, aklystron, a gyrotron, a traveling-wave tube amplifier/oscillator, or anyother source of radiation. Further, in one embodiment, the radiationsources may be cross-polarized.

[0060] Additionally, in one embodiment, the method may includeactivating a plurality of radiation sources, wherein each of theplurality of microwave sources is successively activated atpredetermined intervals.

[0061] In another embodiment, the plasma region may contain a plasmacatalyst. The plasma catalyst consistent with this invention can includeone or more different materials and may be either passive or active. Aplasma catalyst can be used, among other things, to ignite, modulate,and/or sustain a plasma at a gas pressure that is less than, equal to,or greater than atmospheric pressure.

[0062] Plasma Catalysts

[0063] One method of forming a plasma consistent with this invention caninclude subjecting a gas in a cavity to electromagnetic radiation havinga frequency less than about 333 GHz in the presence of a passive plasmacatalyst. A passive plasma catalyst consistent with this invention caninclude any object capable of inducing a plasma by deforming a localelectric field (e.g., an electromagnetic field) consistent with thisinvention, without necessarily adding additional energy through thecatalyst, such as by applying an electric voltage to create a spark.

[0064] A passive plasma catalyst consistent with this invention can alsobe a nano-particle or a nano-tube. As used herein, the term“nano-particle” can include any particle having a maximum physicaldimension less than about 100 nm that is at least electricallysemi-conductive. Also, both single-walled and multi-walled carbonnanotubes, doped and undoped, can be particularly effective for ignitingplasmas consistent with this invention because of their exceptionalelectrical conductivity and elongated shape. The nanotubes can have anyconvenient length and can be a powder fixed to a substrate. If fixed,the nanotubes can be oriented randomly on the surface of the substrateor fixed to the substrate (e.g., at some predetermined orientation)while the plasma is ignited or sustained.

[0065] A passive plasma catalyst can also be a powder consistent withthis invention, and need not comprise nano-particles or nano-tubes. Itcan be formed, for example, from fibers, dust particles, flakes, sheets,etc. When in powder form, the catalyst can be suspended, at leasttemporarily, in a gas. By suspending the powder in the gas, the powdercan be quickly dispersed throughout the cavity and more easily consumed,if desired.

[0066] In one embodiment, the powder catalyst can be carried into thecavity and at least temporarily suspended with a carrier gas. Thecarrier gas can be the same or different from the gas that forms theplasma. Also, the powder can be added to the gas prior to beingintroduced to the cavity. For example, as shown in FIG. 2, radiationsource 52 and radiation source 54 can supply radiation to radiationcavity 55, in which plasma cavity 60 is placed. Powder source 65provides catalytic powder 70 into gas stream 75. In an alternativeembodiment, powder 70 can be first added to cavity 60 in bulk (e.g., ina pile) and then distributed in the cavity in any number of ways,including flowing a gas through or over the bulk powder. In addition,the powder can be added to the gas for igniting, modulating, orsustaining a plasma by moving, conveying, drizzling, sprinkling,blowing, or otherwise, feeding the powder into or within the cavity.Although FIG. 2 shows only two radiation sources, additional radiationsources may be used. In one embodiment consistent with this invention,microwave source 54 may be activated and then, after a plasma is formed,microwave source 55 may be activated.

[0067] In one experiment, a plasma was ignited in a cavity by placing apile of carbon fiber powder in a copper pipe that extended into thecavity. Although sufficient radiation was directed into the cavity, thecopper pipe shielded the powder from the radiation and no plasmaignition took place. However, once a carrier gas began flowing throughthe pipe, forcing the powder out of the pipe and into the cavity, andthereby subjecting the powder to the radiation, a plasma was nearlyinstantaneously ignited in the cavity. This permits a subsequentradiation source to be activated, which reduces the ramp-up timerequired to achieve, for example, very high temperature plasmas.

[0068] A powder plasma catalyst consistent with this invention can besubstantially non-combustible, thus it need not contain oxygen or burnin the presence of oxygen. Thus, as mentioned above, the catalyst caninclude a metal, carbon, a carbon-based alloy, a carbon-based composite,an electrically conductive polymer, a conductive silicone elastomer, apolymer nanocomposite, an organic-inorganic composite, and anycombination thereof.

[0069] Also, powder catalysts can be substantially uniformly distributedin the plasma cavity (e.g., when suspended in a gas), and plasmaignition can be precisely controlled within the cavity. Uniform ignitioncan be important in certain applications, including those applicationsrequiring brief plasma exposures, such as in the form of one or morebursts. Still, a certain amount of time can be required for a powdercatalyst to distribute itself throughout a cavity, especially incomplicated, multi-chamber cavities. Therefore, consistent with anotheraspect of this invention, a powder catalyst can be introduced into thecavity through a plurality of ignition ports to more rapidly obtain amore uniform catalyst distribution therein (see below).

[0070] In addition to powder, a passive plasma catalyst consistent withthis invention can include, for example, one or more microscopic ormacroscopic fibers, sheets, needles, threads, strands, filaments, yarns,twines, shavings, slivers, chips, woven fabrics, tape, whiskers, or anycombination thereof. In these cases, the plasma catalyst can have atleast one portion with one physical dimension substantially larger thananother physical dimension. For example, the ratio between at least twoorthogonal dimensions should be at least about 1:2, but could be greaterthan about 1:5, or even greater than about 1:10.

[0071] Thus, a passive plasma catalyst can include at least one portionof material that is relatively thin compared to its length. A bundle ofcatalysts (e.g., fibers) may also be used and can include, for example,a section of graphite tape. In one experiment, a section of tape havingapproximately thirty thousand strands of graphite fiber, each about 2-3microns in diameter, was successfully used. The number of fibers in andthe length of a bundle are not critical to igniting, modulating, orsustaining the plasma. For example, satisfactory results have beenobtained using a section of graphite tape about one-quarter inch long.One type of carbon fiber that has been successfully used consistent withthis invention is sold under the trademark Magnamite®, Model No.AS4C-GP3K, by the Hexcel Corporation, of Anderson, S.C. Also,silicon-carbide fibers have been successfully used.

[0072] A passive plasma catalyst consistent with another aspect of thisinvention can include one or more portions that are, for example,substantially spherical, annular, pyramidal, cubic, planar, cylindrical,rectangular or elongated.

[0073] The passive plasma catalysts discussed above include at least onematerial that is at least electrically semi-conductive. In oneembodiment, the material can be highly conductive. For example, apassive plasma catalyst consistent with this invention can include ametal, an inorganic material, carbon, a carbon-based alloy, acarbon-based composite, an electrically conductive polymer, a conductivesilicone elastomer, a polymer nanocomposite, an organic-inorganiccomposite, or any combination thereof. Some of the possible inorganicmaterials that can be included in the plasma catalyst include carbon,silicon carbide, molybdenum, platinum, tantalum, tungsten, carbonnitride, and aluminum, although other electrically conductive inorganicmaterials are believed to work just as well.

[0074] In addition to one or more electrically conductive materials, apassive plasma catalyst consistent with this invention can include oneor more additives (which need not be electrically conductive). As usedherein, the additive can include any material that a user wishes to addto the plasma. For example, in doping semiconductors and othermaterials, one or more dopants can be added to the plasma through thecatalyst. See, e.g., commonly owned, concurrently filed U.S. patentapplication Ser. No. ______ (Attorney Docket No. 1837.0026), which ishereby incorporated by reference in its entirety. The catalyst caninclude the dopant itself, or it can include a precursor material that,upon decomposition, can form the dopant. Thus, the plasma catalyst caninclude one or more additives and one or more electrically conductivematerials in any desirable ratio, depending on the ultimate desiredcomposition of the plasma and the process using the plasma.

[0075] The ratio of the electrically conductive components to theadditives in a passive plasma catalyst can vary over time while beingconsumed. For example, during ignition, the plasma catalyst coulddesirably include a relatively large percentage of electricallyconductive components to improve the ignition conditions. On the otherhand, if used while sustaining the plasma, the catalyst could include arelatively large percentage of additives. It will be appreciated bythose of ordinary skill in the art that the component ratio of theplasma catalyst used to ignite and sustain the plasma could be the same.

[0076] A predetermined ratio profile can be used to simplify many plasmaprocesses. In many conventional plasma processes, the components withinthe plasma are added as necessary, but such addition normally requiresprogrammable equipment to add the components according to apredetermined schedule. However, consistent with this invention, theratio of components in the catalyst can be varied, and thus the ratio ofcomponents in the plasma itself can be automatically varied. That is,the ratio of components in the plasma at any particular time can dependon which of the catalyst portions is currently being consumed by theplasma. Thus, the catalyst component ratio can be different at differentlocations within the catalyst. And, the current ratio of components in aplasma can depend on the portions of the catalyst currently and/orpreviously consumed, especially when the flow rate of a gas passingthrough the plasma chamber is relatively slow.

[0077] A passive plasma catalyst consistent with this invention can behomogeneous, inhomogeneous, or graded. Also, the plasma catalystcomponent ratio can vary continuously or discontinuously throughout thecatalyst. For example, in FIG. 3, the ratio can vary smoothly forming agradient along a length of catalyst 100. Catalyst 100 can include astrand of material that includes a relatively low concentration of acomponent at section 105 and a continuously increasing concentrationtoward section 110.

[0078] Alternatively, as shown in FIG. 4, the ratio can varydiscontinuously in each portion of catalyst 120, which includes, forexample, alternating sections 125 and 130 having differentconcentrations. It will be appreciated that catalyst 120 can have morethan two section types. Thus, the catalytic component ratio beingconsumed by the plasma can vary in any predetermined fashion. In oneembodiment, when the plasma is monitored and a particular additive isdetected, further processing can be automatically commenced orterminated.

[0079] Another way to vary the ratio of components in a sustained plasmais by introducing multiple catalysts having different component ratiosat different times or different rates. For example, multiple catalystscan be introduced at approximately the same location or at differentlocations within the cavity. When introduced at different locations, theplasma formed in the cavity can have a component concentration gradientdetermined by the locations of the various catalysts. Thus, an automatedsystem can include a device by which a consumable plasma catalyst ismechanically inserted before and/or during plasma igniting, modulating,and/or sustaining.

[0080] A passive plasma catalyst consistent with this invention can alsobe coated. In one embodiment, a catalyst can include a substantiallynon-electrically conductive coating deposited on the surface of asubstantially electrically conductive material. Alternatively, thecatalyst can include a substantially electrically conductive coatingdeposited on the surface of a substantially electrically non-conductivematerial. FIGS. 5A and 5B, for example, show fiber 140, which includesunderlayer 145 and coating 150. In one embodiment, a plasma catalystincluding a carbon core is coated with nickel to prevent oxidation ofthe carbon.

[0081] A single plasma catalyst can also include multiple coatings. Ifthe coatings are consumed during contact with the plasma, the coatingscould be introduced into the plasma sequentially, from the outer coatingto the innermost coating, thereby creating a time-release mechanism.Thus, a coated plasma catalyst can include any number of materials, aslong as a portion of the catalyst is at least electricallysemi-conductive.

[0082] Consistent with another embodiment of this invention, a plasmacatalyst can be located entirely within a radiation cavity tosubstantially reduce or prevent radiation energy leakage. In this way,the plasma catalyst does not electrically or magnetically couple withthe vessel containing the cavity or to any electrically conductiveobject outside the cavity. This prevents sparking at the ignition portand prevents radiation from leaking outside the cavity during theignition and possibly later if the plasma is sustained. In oneembodiment, the catalyst can be located at a tip of a substantiallyelectrically non-conductive extender that extends through an ignitionport.

[0083]FIG. 6, for example, shows radiation chamber 160 in which plasmacavity 165 is placed. Plasma catalyst 170 is elongated and extendsthrough ignition port 175. As shown in FIG. 7, and consistent with thisinvention, catalyst 170 can include electrically conductive distalportion 180 (which is placed in chamber 160) and electricallynon-conductive portion 185 (which is placed substantially outsidechamber 160). This configuration prevents an electrical connection(e.g., sparking) between distal portion 180 and chamber 160.

[0084] In another embodiment, shown in FIG. 8, the catalyst can beformed from a plurality of electrically conductive segments 190separated by and mechanically connected to a plurality of electricallynon-conductive segments 195. In this embodiment, the catalyst can extendthrough the ignition port between a point inside the cavity and anotherpoint outside the cavity, but the electrically discontinuous profilesignificantly prevents sparking and energy leakage.

[0085] Active Plasma Catalyst

[0086] Another method of forming a plasma consistent with this inventionincludes subjecting a gas in a cavity to electromagnetic radiationhaving a frequency less than about 333 GHz in the presence of an activeplasma catalyst, which generates or includes at least one ionizingparticle.

[0087] An active plasma catalyst consistent with this invention can beany particle or high energy wave packet capable of transferring asufficient amount of energy to a gaseous atom or molecule to remove atleast one electron from the gaseous atom or molecule in the presence ofelectromagnetic radiation. Depending on the source, the ionizingparticles can be directed into the cavity in the form of a focused orcollimated beam, or they may be sprayed, spewed, sputtered, or otherwiseintroduced.

[0088] For example, FIG. 9 shows radiation source 200 and radiationsource 202 for directing radiation into radiation chamber 205. Plasmacavity 210 is positioned inside of chamber 205 and may permit a gas toflow therethrough via ports 215 and 216. Source 220 directs ionizingparticles 225 into cavity 210. Source 220 can be protected, for example,by a metallic screen which allows the ionizing particles to pass throughbut shields source 220 from radiation. If necessary, source 220 can bewater-cooled. In one embodiment, radiation source 200 may be activatedand then, after a plasma is formed, radiation source 202 may beactivated. Alternatively, radiation source 202 may be activated after apredetermined interval.

[0089] Examples of ionizing particles consistent with this invention caninclude x-ray particles, gamma ray particles, alpha particles, betaparticles, neutrons, protons, and any combination thereof. Thus, anionizing particle catalyst can be charged (e.g., an ion from an ionsource) or uncharged and can be the product of a radioactive fissionprocess. In one embodiment, the vessel in which the plasma cavity isformed could be entirely or partially transmissive to the ionizingparticle catalyst. Thus, when a radioactive fission source is locatedoutside the cavity, the source can direct the fission products throughthe vessel to ignite the plasma. The radioactive fission source can belocated inside the radiation chamber to substantially prevent thefission products (i.e., the ionizing particle catalyst) from creating asafety hazard.

[0090] In another embodiment, the ionizing particle can be a freeelectron, but it need not be emitted in a radioactive decay process. Forexample, the electron can be introduced into the cavity by energizingthe electron source (such as a metal), such that the electrons havesufficient energy to escape from the source. The electron source can belocated inside the cavity, adjacent the cavity, or even in the cavitywall. It will be appreciated by those of ordinary skill in the art thatthe any combination of electron sources is possible. A common way toproduce electrons is to heat a metal, and these electrons can be furtheraccelerated by applying an electric field.

[0091] In addition to electrons, free energetic protons can also be usedto catalyze a plasma. In one embodiment, a free proton can be generatedby ionizing hydrogen and, optionally, accelerated with an electricfield.

[0092] Multi-Mode Radiation Cavities

[0093] A radiation waveguide, cavity, or chamber can support orfacilitate propagation of at least one electromagnetic radiation mode.As used herein, the term “mode” refers to a particular pattern of anystanding or propagating electromagnetic wave that satisfies Maxwell'sequations and the applicable boundary conditions (e.g., of the cavity).In a waveguide or cavity, the mode can be any one of the variouspossible patterns of propagating or standing electromagnetic fields.Each mode is characterized by its frequency and polarization of theelectric field and/or the magnetic field vectors. The electromagneticfield pattern of a mode depends on the frequency, refractive indices ordielectric constants, and waveguide or cavity geometry.

[0094] A transverse electric (TE) mode is one whose electric fieldvector is normal to the direction of propagation. Similarly, atransverse magnetic (TM) mode is one whose magnetic field vector isnormal to the direction of propagation. A transverse electric andmagnetic (TEM) mode is one whose electric and magnetic field vectors areboth normal to the direction of propagation. A hollow metallic waveguidedoes not typically support a normal TEM mode of radiation propagation.Even though radiation appears to travel along the length of a waveguide,it may do so only by reflecting off the inner walls of the waveguide atsome angle. Hence, depending upon the propagation mode, the radiation(e.g., microwave) may have either some electric field component or somemagnetic field component along the axis of the waveguide (often referredto as the z-axis).

[0095] The actual field distribution inside a cavity or waveguide is asuperposition of the modes therein. Each of the modes can be identifiedwith one or more subscripts (e.g., TE₁₀ (“tee ee one zero”). Thesubscripts normally specify how many “half waves” at the guidewavelength are contained in the x and y directions. It will beappreciated by those skilled in the art that the guide wavelength can bedifferent from the free space wavelength because radiation propagatesinside the waveguide by reflecting at some angle from the inner walls ofthe waveguide. In some cases, a third subscript can be added to definethe number of half waves in the standing wave pattern along the z-axis.

[0096] For a given radiation frequency, the size of the waveguide can beselected to be small enough so that it can support a single propagationmode. In such a case, the system is called a single-mode system (i.e., asingle-mode applicator). The TE₁₀ mode is usually dominant in arectangular single-mode waveguide.

[0097] As the size of the waveguide (or the cavity to which thewaveguide is connected) increases, the waveguide or applicator cansometimes support additional higher order modes forming a multi-modesystem. When many modes are capable of being supported simultaneously,the system is often referred to as highly moded.

[0098] A simple, single-mode system has a field distribution thatincludes at least one maximum and/or minimum. The magnitude of a maximumlargely depends on the amount of radiation supplied to the system. Thus,the field distribution of a single mode system is strongly varying andsubstantially non-uniform.

[0099] Unlike a single-mode cavity, a multi-mode cavity can supportseveral propagation modes simultaneously, which, when superimposed,results in a complex field distribution pattern. In such a pattern, thefields tend to spatially smear and, thus, the field distribution usuallydoes not show the same types of strong minima and maxima field valueswithin the cavity. In addition, as explained more fully below, amode-mixer can be used to “stir” or “redistribute” modes (e.g., bymechanical movement of a radiation reflector). This redistributiondesirably provides a more uniform time-averaged field distributionwithin the cavity.

[0100] A multi-mode cavity consistent with this invention can support atleast two modes, and may support many more than two modes. Each mode hasa maximum electric field vector. Although there may be two or moremodes, one mode may be dominant and has a maximum electric field vectormagnitude that is larger than the other modes. As used herein, amulti-mode cavity may be any cavity in which the ratio between the firstand second mode magnitudes is less than about 1:10, or less than about1:5, or even less than about 1:2. It will be appreciated by those ofordinary skill in the art that the smaller the ratio, the moredistributed the electric field energy between the modes, and hence themore distributed the radiation energy is in the cavity.

[0101] The distribution of plasma within a processing cavity maystrongly depend on the distribution of the applied radiation. Forexample, in a pure single mode system, there may only be a singlelocation at which the electric field is a maximum. Therefore, a strongplasma may only form at that single location. In many applications, sucha strongly localized plasma could undesirably lead to non-uniform plasmatreatment or heating (i.e., localized overheating and underheating).

[0102] Whether or not a single or multi-mode cavity is used consistentwith this invention, it will be appreciated by those of ordinary skillin the art that the cavity in which the plasma is formed can becompletely closed or partially open. For example, in certainapplications, such as in plasma-assisted furnaces, the cavity could beentirely closed. See, for example, commonly owned, concurrently filedU.S. patent application Ser. No. ______ (Attorney Docket No. 1837.0020),which is fully incorporated herein by reference. In other applications,however, it may be desirable to flow a gas through the cavity, andtherefore the cavity must be open to some degree. In this way, the flow,type, and pressure of the flowing gas can be varied over time. This maybe desirable because certain gases with lower ionization potentials,such as argon, are easier to ignite but may have other undesirableproperties during subsequent plasma processing.

[0103] Mode-Mixing

[0104] For many applications, a cavity containing a uniform plasma isdesirable. However, because microwave radiation can have a relativelylong wavelength (e.g., several tens of centimeters), obtaining a uniformdistribution can be difficult to achieve. As a result, consistent withone aspect of this invention, the radiation modes in a multi-mode cavitycan be mixed, or redistributed, over a period of time. Because the fielddistribution within the cavity must satisfy all of the boundaryconditions set by the inner surface of the cavity, those fielddistributions can be changed by changing the position of any portion ofthat inner surface.

[0105] In one embodiment consistent with this invention, a movablereflective surface can be located inside the radiation cavity. The shapeand motion of the reflective surface should, when combined, change theinner surface of the cavity during motion. For example, an “L” shapedmetallic object (i.e., “mode-mixer”) when rotated about any axis willchange the location or the orientation of the reflective surfaces in thecavity and therefore change the radiation distribution therein. Anyother asymmetrically shaped object can also be used (when rotated), butsymmetrically shaped objects can also work, as long as the relativemotion (e.g., rotation, translation, or a combination of both) causessome change in the location or the orientation of the reflectivesurfaces. In one embodiment, a mode-mixer can be a cylinder that isrotatable about an axis that is not the cylinder's longitudinal axis.

[0106] Each mode of a multi-mode cavity may have at least one maximumelectric field vector, but each of these vectors could occurperiodically across the inner dimension of the cavity. Normally, thesemaxima are fixed, assuming that the frequency of the radiation does notchange. However, by moving a mode-mixer such that it interacts with theradiation, it is possible to move the positions of the maxima. Forexample, mode-mixer 38 can be used to optimize the field distributionwithin cavity 14 such that the plasma ignition conditions and/or theplasma sustaining conditions are optimized. Thus, once a plasma isexcited, the position of the mode-mixer can be changed to move theposition of the maxima for a uniform time-averaged plasma process (e.g.,heating).

[0107] Thus, consistent with this invention, mode-mixing can be usefulduring plasma ignition. For example, when an electrically conductivefiber is used as a plasma catalyst, it is known that the fiber'sorientation can strongly affect the minimum plasma-ignition conditions.It has been reported, for example, that when such a fiber is oriented atan angle that is greater than 60° to the electric field, the catalystdoes little to improve, or relax, these conditions. By moving areflective surface either in or near the cavity, however, the electricfield distribution can be significantly changed.

[0108] Mode-mixing can also be achieved by launching the radiation intothe applicator chamber through, for example, a rotating waveguide jointthat can be mounted inside the applicator chamber. The rotary joint canbe mechanically moved (e.g., rotated) to effectively launch theradiation in different directions in the radiation chamber. As a result,a changing field pattern can be generated inside the applicator chamber.

[0109] Mode-mixing can also be achieved by launching radiation in theradiation chamber through a flexible waveguide. In one embodiment, thewaveguide can be mounted inside the chamber. In another embodiment, thewaveguide can extend into the chamber. The position of the end portionof the flexible waveguide can be continually or periodically moved(e.g., bent) in any suitable manner to launch the radiation (e.g.,microwave radiation) into the chamber at different directions and/orlocations. This movement can also result in mode-mixing and facilitatemore uniform plasma processing (e.g., heating) on a time-averaged basis.Alternatively, this movement can be used to optimize the location of aplasma for ignition or other plasma-assisted process.

[0110] If the flexible waveguide is rectangular, a simple twisting ofthe open end of the waveguide will rotate the orientation of theelectric and the magnetic field vectors in the radiation inside theapplicator chamber. Then, a periodic twisting of the waveguide canresult in mode-mixing as well as rotating the electric field, which canbe used to assist ignition, modulation, or sustaining of a plasma.

[0111] Thus, even if the initial orientation of the catalyst isperpendicular to the electric field, the redirection of the electricfield vectors can change the ineffective orientation to a more effectiveone. Those skilled in the art will appreciate that mode-mixing can becontinuous, periodic, or preprogrammed.

[0112] In addition to plasma ignition, mode-mixing can be useful duringsubsequent plasma processing to reduce or create (e.g., tune) “hotspots” in the chamber. When a microwave cavity only supports a smallnumber of modes (e.g., less than 5), one or more localized electricfield maxima can lead to “hot spots” (e.g., within cavity 12). In oneembodiment, these hot spots could be configured to coincide with one ormore separate, but simultaneous, plasma ignitions or processing events.Thus, the plasma catalyst can be located at one or more of thoseignition or subsequent processing positions.

[0113] Multi-Location Ignition

[0114] A plasma can be ignited using multiple plasma catalysts atdifferent locations. In one embodiment, multiple fibers can be used toignite the plasma at different points within the cavity. Suchmulti-point ignition can be especially beneficial when a uniform plasmaignition is desired. For example, when a plasma is modulated at a highfrequency (i.e., tens of Hertz and higher), or ignited in a relativelylarge volume, or both, substantially uniform instantaneous striking andrestriking of the plasma can be improved. Alternatively, when plasmacatalysts are used at multiple points, they can be used to sequentiallyignite a plasma at different locations within a plasma chamber byselectively introducing the catalyst at those different locations. Inthis way, a plasma ignition gradient can be controllably formed withinthe cavity, if desired.

[0115] Also, in a multi-mode cavity, random distribution of the catalystthroughout multiple locations in the cavity increases the likelihoodthat at least one of the fibers, or any other passive plasma catalystconsistent with this invention, is optimally oriented with the electricfield lines. Still, even where the catalyst is not optimally oriented(not substantially aligned with the electric field lines), the ignitionconditions are improved.

[0116] Furthermore, because a catalytic powder can be suspended in agas, it is believed that each powder particle may have the effect ofbeing placed at a different physical location within the cavity, therebyimproving ignition uniformity within the cavity.

[0117] Dual-Cavity Plasma Igniting/Sustaining

[0118] A dual-cavity arrangement can be used to ignite and sustain aplasma consistent with this invention. In one embodiment, a systemincludes at least a first ignition cavity and a second cavity in fluidcommunication with the first cavity. To ignite a plasma, a gas in thefirst ignition cavity can be subjected to electromagnetic radiationhaving a frequency less than about 333 GHz, optionally in the presenceof a plasma catalyst. In this way, the proximity of the first and secondcavities may permit a plasma formed in the first cavity to ignite aplasma in the second cavity, which may be sustained with additionalelectromagnetic radiation.

[0119] In one embodiment of this invention, the first cavity can be verysmall and designed primarily, or solely for plasma ignition. In thisway, very little microwave energy may be required to ignite the plasma,permitting easier ignition, especially when a plasma catalyst is usedconsistent with this invention.

[0120] In one embodiment, the first cavity may be a substantially singlemode cavity and the second cavity is a multi-mode cavity. When the firstignition cavity only supports a single mode, the electric fielddistribution may strongly vary within the cavity, forming one or moreprecisely located electric field maxima. Such maxima are normally thefirst locations at which plasmas ignite, making them ideal points forplacing plasma catalysts. It will be appreciated, however, that when aplasma catalyst is used, it need not be placed in the electric fieldmaximum and, many cases, need not be oriented in any particulardirection.

[0121] In the foregoing described embodiments, in some instances,various features may be grouped together in a single embodiment forpurposes of streamlining the disclosure. This method of disclosure isnot to be interpreted as reflecting an intention that the claimedinvention requires more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive aspects lie inless than all features of a single foregoing disclosed embodiment. Thus,the following claims are hereby incorporated into this DetailedDescription of Embodiments, with each claim standing on its own as aseparate preferred embodiment of the invention.

We claim:
 1. A radiation apparatus, comprising: a radiation cavity; afirst radiation source for directing electromagnetic radiation having afrequency less than about 333 GHz into the cavity to facilitateformation of plasma in the cavity; a second radiation source fordirecting radiation into the cavity; and a controller for sequentiallyactivating the second radiation source after the first radiation sourceis activated.
 2. The apparatus of claim 1, further comprising at leastone additional radiation source, and wherein the controller isconfigured to activate the additional radiation source only after atleast one of the first and second radiation sources is activated.
 3. Theapparatus of claim 1, further comprising a plurality of additionalradiation sources, and wherein the controller is configured to activateeach of the plurality of additional radiation sources only after atleast one of the first and second radiation sources is activated.
 4. Theapparatus of claim 1, wherein the controller delays activation of thesecond radiation source for a predetermined period following activationof the first radiation source.
 5. The apparatus of claim 1, furthercomprising a detector that provides an indication of radiationabsorption, and wherein the controller delays activation of the secondradiation source until after the controller receives a signal from thedetector that a predetermined absorption level has been reached.
 6. Theapparatus of claim 1, wherein the first radiation source is configuredto generate radiation that is cross-polarized relative to radiationgenerated by the second radiation source.
 7. The apparatus of claim 1,wherein each of the first radiation source and the second radiationsource comprises at least one of a magnetron, a klystron, a gyrotron, atraveling-wave tube amplifier, and any other source of radiation.
 8. Theapparatus of claim 1, further comprising a catalyst located proximatethe cavity, for cooperating with the radiation to cause the gas to forma plasma.
 9. The apparatus of claim 8, wherein the catalyst is at leastone of an active catalyst and a passive catalyst.
 10. The method ofclaim 9, wherein the catalyst comprises at least one of metal, inorganicmaterial, carbon, carbon-based alloy, carbon-based composite,electrically conductive polymer, conductive silicone elastomer, polymernanocomposite, and an organic-inorganic composite.
 11. The method ofclaim 10, wherein the catalyst is in the form of at least one of anano-particle, a nano-tube, a powder, a dust, a flake, a fiber, a sheet,a needle, a thread, a strand, a filament, a yarn, a twine, a shaving, asliver, a chip, a woven fabric, a tape, and a whisker.
 12. The method ofclaim 11, wherein the catalyst comprises carbon fiber.
 13. The method ofclaim 9, wherein the catalyst is in the form of at least one of anano-particle, a nano-tube, a powder, a dust, a flake, a fiber, a sheet,a needle, a thread, a strand, a filament, a yarn, a twine, a shaving, asliver, a chip, a woven fabric, a tape, and a whisker.
 14. The apparatusof claim 1, further comprising an isolator for protectively separatingthe second radiation source from the first radiation source.
 15. Aplasma apparatus, comprising: a chamber; a conduit for supplying a gasto the chamber; a plurality of radiation sources arranged to radiateradiation into the chamber; and a controller for delaying activation ofall but a first of the plurality radiation sources until after the firstradiation source is activated.
 16. The plasma apparatus of claim 15,wherein the controller delays activation of at least one of theplurality of radiation sources for a predetermined period followingactivation of the first radiation source and wherein each of theremaining plurality of radiation sources is successively activated atpredetermined intervals.
 17. The plasma apparatus of claim 15, furthercomprising a detector that provides an indication of radiationabsorption, and wherein the controller delays activation of each one ofthe plurality of radiation sources until after the controller receives asignal from the detector that a predetermined absorption level has beenreached.
 18. The plasma apparatus of claim 15, wherein the firstradiation source is configured to generate radiation that iscross-polarized relative to radiation generated by at least one of theplurality of radiation sources.
 19. The plasma apparatus of claim 15,wherein each of the first radiation source and each one of the pluralityradiation sources comprises at least one of a magnetron, a klystron, agyrotron, a traveling-wave tube amplifier/oscillator, and any othersource of radiation.
 20. The plasma apparatus of claim 15, furthercomprising a catalyst located proximate the cavity, for cooperating withthe radiation to cause the gas to form a plasma.
 21. The plasmaapparatus of claim 20, wherein the catalyst is at least one of an activecatalyst and a passive catalyst.
 22. The plasma apparatus of claim 21,wherein the catalyst comprises at least one of metal, inorganicmaterial, carbon, carbon-based alloy, carbon-based composite,electrically conductive polymer, conductive silicone elastomer, polymernanocomposite, and an organic-inorganic composite.
 23. The plasmaapparatus of claim 22, wherein the catalyst is in the form of at leastone of a nano-particle, a nano-tube, a powder, a dust, a flake, a fiber,a sheet, a needle, a thread, a strand, a filament, a yarn, a twine, ashaving, a sliver, a chip, a woven fabric, a tape, and a whisker. 24.The plasma apparatus of claim 23, wherein the catalyst comprises carbonfiber.
 25. The plasma apparatus of claim 21, wherein the catalyst is inthe form of at least one of a nano-particle, a nano-tube, a powder, adust, a flake, a fiber, a sheet, a needle, a thread, a strand, afilament, a yarn, a twine, a shaving, a sliver, a chip, a woven fabric,a tape, and a whisker.
 26. The plasma apparatus of claim 21 wherein theplasma catalyst comprises an active plasma catalyst including at leastone ionizing particle.
 27. The plasma apparatus of claim 26, wherein theat least one ionizing particle comprises a beam of particles.
 28. Theplasma apparatus of claim 26, wherein the particle is at least one of anx-ray particle, a gamma ray particle, an alpha particle, a betaparticle, a neutron, and a proton.
 29. The plasma apparatus of claim 26,wherein the at least one ionizing particle is a charged particle. 30.The plasma apparatus of claim 26, wherein the ionizing particlecomprises a radioactive fission product.
 31. The plasma apparatus ofclaim 15, further comprising a plurality of isolators for protectivelyseparating each one of the plurality of radiation sources from eachother and from the first radiation source.
 32. The plasma apparatus ofclaim 15, wherein the chamber is a waveguide.
 33. A method employing atleast first and second radiation sources both arranged to directradiation into a plasma region, the method comprising: introducing gasinto the plasma region; activating the first radiation source in orderto facilitate formation of plasma in the plasma region; and activatingthe second radiation source after the plasma is formed.
 34. The methodof claim 33, wherein formation of plasma is facilitated using at leastone of a pointed metal tip, a spark generator, carbon, fiberousmaterial, powderous material and any other catalyst capable of causingplasma ignition.
 35. The method of claim 33, further comprisingactivating at least one additional radiation source after at least oneof the first and second sources is activated.
 36. The method of claim33, further comprising delaying activation of the at least oneadditional radiation source until after both the first and secondradiation sources are activated.
 37. The method of claim 33, whereinmicrowave radiation from the first radiation source is cross-polarizedrelative to radiation from the second radiation source.
 38. The methodof claim 33, further comprising activating a plurality of radiationsources, wherein each of the plurality of radiation sources issuccessively activated at predetermined intervals.
 39. The method ofclaim 33, wherein each of the first radiation source and the secondradiation source comprises at least one of a magnetron, a klystron, agyrotron, a traveling-wave tube amplifier, and any other source ofradiation.
 40. The method of claim 33, wherein the heating regioncontains a plasma catalyst.
 41. The method of claim 33, wherein theactivating the first microwave source comprises: igniting the plasma bysubjecting the gas in the region to electromagnetic radiation generatedby the first source having a frequency less than about 333 GHz in thepresence of at least one passive plasma catalyst comprising a materialthat is at least electrically semi-conductive.
 42. The method of claim41, wherein the material comprises at least one of metal, inorganicmaterial, carbon, carbon-based alloy, carbon-based composite,electrically conductive polymer, conductive silicone elastomer, polymernanocomposite, organic-inorganic composite, and any combination thereof.43. The method of claim 41, wherein the material is in the form of atleast one of a nano-particle, a nano-tube, a powder, a dust, a flake, afiber, a sheet, a needle, a thread, a strand, a filament, a yarn, atwine, a shaving, a sliver, a chip, a woven fabric, a tape, a whisker,and any combination thereof.
 44. The method of claim 41, wherein the atleast one passive plasma catalyst comprises a plurality of elongated,electrically conductive items distributed in differing locations in thecavity.
 45. The method of claim 41, wherein the region is located in acavity configured to support at least a first mode and a second mode ofthe electromagnetic radiation, each of the modes having a maximumelectric field vector in the cavity, each of the vectors having amagnitude, and wherein a ratio between the first mode magnitude and thesecond mode magnitude is less than about 1:10.
 46. The method of claim45, wherein the ratio is less than about 1:5.
 47. The method of claim45, wherein the ratio is less than about 1:2.
 48. The method of claim33, wherein the activating the first radiation source comprises:subjecting a gas in a cavity to electromagnetic radiation having afrequency less than about 333 GHz in the presence of an active plasmacatalyst comprising at least one ionizing particle.
 49. The method ofclaim 48, wherein the at least one ionizing particle comprises a beam ofparticles.
 50. The method of claim 48, wherein the particle is at leastone of an x-ray particle, a gamma ray particle, an alpha particle, abeta particle, a neutron, and a proton.
 51. The method of claim 48,wherein the at least one ionizing particle is a charged particle. 52.The method of claim 48, wherein the ionizing particle comprises aradioactive fission product.
 53. The method of claim 52, wherein acavity is formed in a vessel that is at least partially transmissive tothe product, the method further comprising positioning a radioactivefission source outside the cavity such that the source directs thefission product through the vessel into the cavity.
 54. The method ofclaim 52, wherein the vessel and the radioactive fission source areinside a radiation chamber, and wherein the chamber comprises a materialthat substantially prevents the product from escaping the chamber. 55.The method of claim 52 further comprising positioning a radioactivefission source in a cavity, wherein the source generates the at leastone fission product.
 56. The method of claim 48, wherein the ionizingparticle is a free electron, the method further comprising generatingthe electron by energizing an electron source.
 57. The method of claim56, wherein the energizing comprises heating the electron source. 58.The method of claim 48, wherein the particle comprises a free proton,the method further comprising generating the free proton by ionizinghydrogen.
 59. The method of claim 48, wherein the cavity is at leastpartially open, permitting the gas to flow therethrough.